1. Field of the Invention
The present invention relates to a multi-cladding optical fiber that is suited to the transmission of high-power light, an optical fiber module that has the optical fiber, a fiber laser and a fiber amplifier.
2. Description of the Related Art
In recent years, there have been dramatic advances in the output power level of fiber amplifiers and fiber lasers. Accompanying these advances in increased output power, improvements in endurance to high-power light have been desired for various optical fiber components such as rare earth-doped optical fibers that are applied to fiber amplifiers and fiber lasers. When considering the endurance of optical fibers to high-power light, it is important to suppress the influences imparted by optical damage and nonlinear optical effects. Optical damage and nonlinear optical effects are both phenomena that occur when the optical power density (the optical, power per unit area of the light propagation cross section) is high. Accordingly, in order to obtain a high output power while suppressing the occurrence of optical damage and nonlinear optical effects, the optical power density should be lowered. In order to lower the optical power density without lowering the output power, it is a promising candidate to increase the cross-sectional area through which light passes (light propagation cross-sectional area). Usually, the effective core cross-sectional area (Aeff) that is defined by Equation (1) given below is used as an indicator.
                              [                      Equation            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                A          eff                =                              2            ⁢                                          π                ⁡                                  [                                                            ∫                      0                      ∞                                        ⁢                                                                                                                                                  E                            ⁡                                                          (                              r                              )                                                                                                                                2                                            ⁢                      r                      ⁢                                                                                          ⁢                                              ⅆ                        r                                                                              ]                                            2                                                          ∫              0              ∞                        ⁢                                                                                                  E                    ⁡                                          (                      r                      )                                                                                        4                            ⁢              r              ⁢                                                          ⁢                              ⅆ                r                                                                        (        1        )            
In Equation (1), E(r) indicates the electric field distribution of light in the optical fiber, and r indicates the distance in the radial direction from the axial center of the optical fiber.
From such a viewpoint, a method of enlarging the effective core cross-sectional area has been actively studied in recent years.
For example, Non-patent Document 1 (Proceedings of the SPIE, Vol. 5335, pp. 132-139) discloses a method of enlarging the effective core cross-sectional area by changing the refractive-index profile of the optical fiber core.
Non-patent Document 2 (Optics Letters, Vol. 25, pp. 442-444 (2000)) discloses a method of substantially realizing single-mode propagation in a multimode optical fiber in which the effective core cross-sectional area is large by bending the optical fiber to induce bending loss in the high-order modes selectively, even in an optical fiber in which high-order modes exist.
Non-patent Document 3 (Optics Express, Vol. 14, pp. 2715-2720 (2006)) and Non-patent Document 4 (Proceedings of the ECOC 2008, Th. 3. C. 1 (2008)) respectively disclose a method of effective core cross-sectional area enlargement that uses a photonic crystal fiber (PCF) structure, and a method of effective core cross-sectional area enlargement by reducing the relative refractive index difference.
Non-patent Document 5 (Proceedings of CLEO/QELS 2008, CPDB6 (2008)) discloses a method of effective core cross-sectional area enlargement that uses a leakage fiber.
Non-patent Document 6 (Proceedings of OFC/NFOEC 2008, OWU2 (2008)), Non-patent Document 7 (Optics Express, 13, pp. 3477-3490 (2005)), and Non-patent Document 8 (Proceedings of ECAC 2008, Mo. 4. B. 4 (2008)) disclose a method of coupling the higher-order modes, and removing only them around a core, and substantially realizing a single mode propagation.
However, in the method disclosed in Non-patent Document 1, the cut-off wavelength ends up being lengthened together with the enlargement of the effective core cross-sectional area, and giving rise to the problem of a trade-off occurring between the single-mode propagation required for maintaining beam quality and effective core cross-sectional area enlargement. Moreover, in the refractive index profile of the optical fiber that is disclosed in Non-patent Document 1, there has also been the problem of the effective core cross-sectional area becoming significantly smaller when the fiber is used in a bent condition (a detailed study result is given in Optical Express, 14, pp. 69-81 (2006) in relation to the behavior of the effective core cross-sectional area when bent).
Also, the method that is disclosed in Non-patent Document 2 has been comparatively widely used recently, but as disclosed in Proceedings of OFC/NFOEC 2008, OTuJ2 (2008) (hereinafter abbreviated as Non-patent Document 9), there have been the problems of being the presence of a limit in enlargement of the effective core cross-sectional area affected by a reduction in the effective core cross-sectional area when bent, and as a result, it is impossible to sufficiently enlarge the effective core cross-sectional area, and in consideration of a bent condition, the outer diameter of the core being essentially limited to around 25 μm (320 μm2 when converted to the effective core cross-sectional area).
Also, in the methods disclosed in Non-patent Documents 3 and 4, since these fiber structures are sensitive to bending, it cannot be used in a bent condition, and so the problem arises of not being able to realize a compact fiber amplifier and fiber laser.
The method disclosed in Non-patent Document 5 is the same as the method disclosed in Non-patent Documents 3 and 4 on the point of the leakage fiber being sensitive to bending, and since the transmission loss is principally large, there is the problem of raising the lasing efficiency of the laser or amplifying efficiency of the amplifier being difficult.
The methods disclosed in Non-patent Documents 6 to 8 can effectively remove higher-order modes, but the refractive index profile and fiber structure is extremely complicated, and moreover since extremely high precision controls of refractive index profile and fiber structure are required, there have been the problems that fiber manufacturing is difficult, as a result, the manufacturing costs become high, and the yields become low.
The present invention was achieved in view of the aforementioned circumstances, and has an object of providing an optical fiber having a simple structure and capable of substantial single-mode propagation and enlargement of the effective core cross-sectional area.